Project Summary Crucial to any pathogen's replication and spread is an ability to avoid the host's immune responses. An understanding of the molecular mechanisms by which pathogens accomplish this avoidance can be key to the design of treatment and prevention strategies, as well as giving insight into the pathogen response. Kaposi's sarcoma-associated herpesvirus (KSHV), a human pathogen responsible for a large percentage of virally-caused cancers, has evolved a wide variety of strategies for evading or controlling the host immune responses and achieving a life-long, persistent infection. The Mir1 and Mir2 proteins act in concert to remove a number of immunomodulatory proteins from the surface of expressing cells, including MHC class I, B7.2 and ICAM-1. This down regulation results in escape from cytotoxic T lymphocyte and natural killer cell lysis, plus decreased activation of T helper cells. Our lab has discovered two additional targets of regulation, DC-SIGN (CD209) and the related DC-SIGNR. The potential benefits of this down regulation to the biology of the virus are multiple and include an alteration of viral antigen presentation, as well as a potential skewing of the Th1 vs. Th2 immune responses. We also have data demonstrating that both of the SIGN molecules are playing a role in KSHV entry, acting as receptors or accessory molecules. This ability has important implications with regards to the viral transmission, dissemination and pathology. These possibilities will be explored through the following Specific Aims: Specific Aim 1: Molecular mechanisms of KSHV Mir1 and Mir2 modulation of DC-SIGN and DC-SIGNR. Specific Aim 2: The role of DC-SIGN and DC-SIGNR in KSHV infection and pathogenicity. Given the growing numbers of pathogens that utilize DC-SIGN or DC-SIGNR, these experiments will provide important insights into the importance of the SIGN molecules to host immune responses and identify key vulnerabilities that pathogens can exploit. Further, these studies will directly influence the current prophylaxis and treatment stratagies used in KSHV-positive patients and in the prevention of KSHV spread. . Project Narrative: The Aims of this proposal are to understand the molecular mechanisms by which the Mir1 and Mir2 proteins of Kaposi's sarcoma-associated herpesvirus (KSHV) function in the down regulation of DC-SIGN and DC-SIGNR. Because the SIGN molecules are critical in mounting an effective immune response, this proposal should contribute significantly to understanding the ability of KSHV to avoid the host immune responses, persist and drive oncogenesis.